A major source of interest has been to reduce the cost and complication of the assembly of electro-optic devices through the coupling of the light into an external waveguide or other media. The desire to effectively couple light has lead to the development of vertically-emitting (surface-coupled) diodes (as opposed to edge-emitting diodes). The term “vertical” is used in the industry generally for any light output through the top and/or bottom surfaces, including, for example, light coming out at 45 degrees from the vertical. While these chips generate light horizontally (parallel to the top surface), they use gratings to change the direction of the light and couple light out top and/or bottom surfaces. The term “light” as used herein, includes not only visible light, but also infrared and ultraviolet. The term “laser” is used herein to describe light generating devices having an electrically or optically pumped active-region, including devices using two reflectors that form ends of an optical cavity and optical devices that accept a light waveform input and have an amplified light waveform as an output. Lasers generally amplify the light that is allowed to resonate in the cavity. The term “diode” is generally used herein to mean an electrically-pumped, laser chip.
In addition to the horizontal-cavity edge-emitting type of laser, there are vertical-cavity, vertically-emitting laser chips, i.e., the vertical-cavity surface emitting laser, or VCSEL. VCSELs, however, have had substantially reduced performance and a complicated device structure that does not effectively translate across the different material systems (such as GaAs to InP) for low cost manufacturing. The gain volume for VCSEL is very small and thus the output power is low. Note that VCSELs, like edge-emitters, bring light directly out, without diffracting the light.
The need for better vertically-emitting structures has driven the industry to examine a wide number of methods to couple light vertically out of a horizontal cavity structure. Proposed structures include the use of gratings (see, e.g., U.S. Pat. No. 6,219,369 to Portnoi, et al., which uses a single diode on a chip and U.S. Pat. No. 5,673,284 to Congdon, et al., which uses four stripe diodes on a chip). The classic approach to grating coupled devices is to utilize a surface blazed grating with fingers extending down into the surface of a cladding over the passive region to couple light from an active region (containing, e.g., a quantum well, a p-n homojunction or a double heterostructure) through the passive region, and then vertically out of the device. A typical such vertically-emitting laser might have an active region about 10 microns wide by 500 microns long, and two Bragg gratings as end-of cavity-reflectors, and an output grating designed both to couple light out and to reflect light to the active region as the feedback (generally about 70–90% coupled out and 10–30% fed back to give the desired narrow-band emission).